1. Field of the Invention
The present invention relates to both a recording medium disc (as will be shortly referred to as the "disc") such as a video disc or an audio disc, which is formed with a stack rib for protecting the signal face of a disc base or its face having no protecting coating from being damaged, and a mold for molding the disc.
2. Description of the Prior Art
In the case of the prior art, in which a disc base is to be formed by an injection molding, a cavity formed by both the mirror surface of a stationary mold half and a stamper fixed on the mirror surface of a movable mold half by a stamper holder is charged with a molten resin which is injected through a sprue bush.
The disc base thus molded is formed at its center with an opening. In the surface of this disc base, there are formed the helical or concentric pits which are transferred by the stamper. The disc base is formed with an annular groove inside of the signal area having the pits by the stamper holder.
The disc base thus molded is required for recording clear signals to have a low double refractive index but no difference in the double refractive index between the outer and inner circumferential portions of the disc, that is, to have a homogeneous double refractive index so that it may have no internal distortion to enjoy a homogeneous surface finish.
In order to satisfy these requirements, the molten resin has to flow smoothly in the cavity, when this cavity is to be charged with the molten resin, and the air in the cavity has to be discharged to the outside by the molten resin, so that neither the air nor the resin flow pattern may be mixed into the resin to prevent the defective molding.
Here, the disc base is formed with the stack rib for preventing itself, especially its signal area from being damaged due to the contact of stacked disc bases especially when they are stacked just after molded. This stack rib is formed into an annular ridge and so positioned on the side of the disc base, in which no signal is to be recorded, and around the clamp area that it may not fall in the annular groove formed in the opposite face by the stamper holder.
FIG. 6 is a section showing one example of the so-called "compact disc (as will be shortly referred to as the "CD")", which is formed with a stack rib 100 of the prior art. This stack rib 100 is so formed inside of an annular groove 3, which is formed during the molding by the stamper holder, that it may not fall in the annular groove 3. The stack rib 100 is integrally formed with: a top 101 having a height of about 0.22 mm and a flat portion width W1 of about 0.3 mm; a sloped portion 102 leading to a clamp area 6 inside of the top 101; and a sloped portion 103 leading to a signal area 5 outside of the top 101. These sloped portions 102 and 103 have substantially equal angles of slope.
In the mold for forming the aforementioned stack rib, however, the cavity to be formed by clamping the stationary half and the movable half is eaten away by a stamper holder for fixing the stamper and is formed with an annular groove for forming the stack rib. If, therefore, the injection rate is increased, the air is stagnated at the stepped portion of the stamper holder to cause a cold mark. Thus, there arises a problem that the cold mark deteriorates the transferring properties or the double refractive index or that the cold mark extends as far as the signal area so that the disc base cannot be used in its entirety.
In order to prevent the aforementioned cold mark, on the other hand, it is conceivable to lower the injection rate. At the lower injection rate, each cycle takes the longer time to raise a problem that the production efficiency is accordingly lowered.
FIG. 7 is a top plan view showing the cold mark CM which is formed when the disc formed with the stack rib 100 shown in FIG. 6 is molded. This cold mark was formed for a resin charging time of less than 0.60 seconds. In order to eliminate this cold mark, the resin charging time had to be set to 0.70 seconds or longer.
In case, moreover, the molded disc bases are to be stacked, the disc base, which has not been cooled down completely, may have its stack rib stuck on another. When the disc base is to be transferred by means of a suction pad, there arises another problem of the so-called "double suction", in which another disc base sticks to that which is to be transferred, so that two or more disc bases are simultaneously taken out.
In order to solve these problems, the prior art has provided the following counter-measures.
First of all, here will be described one example of the stack rib of the prior art for solving the aforementioned problems. According to this example, the top 111 of a stack rib 110 is made narrower, as shown in FIG. 8, than the top 101 of the stack rib 100 of the prior art to eliminate the flat portion, so that no two disc bases may not be stuck on each other and moved together when they are to be moved one by one by the suction pad or the like from the stack of the molded disc bases.
According to this example, the vertical stick can be eliminated, but the cold marks are liable to occur double or triply, as shown in FIG. 9, and the silver blister is also liable to occur. In order to mold the disc bases stably, therefore, the resin charging time cannot be made shorter than 0.80 seconds.
Here will be described another example of improving the stack rib of the prior art. According to this example, as shown in FIG. 10, the flat top of a stack rib 120 is given such a width of about 1.1 mm as is larger than that of the flat portion of the top 101 of the stack lib 100 of the prior art.
According to this example, however, the cold mark CM formed has a large shape independently of the charging time, as shown in FIG. 11. As a result, the disc base having the stack rib 120 of this example cannot be used in the least.
The present invention has been conceived in view of the problems described above and has an object to provide a disc with a stack rib, which has an excellent production efficiency and an excellent double refractive index.
Another object of the present invention is to provide a mold for molding a disc with a stack rib, which is not required to lower a resin injection rate but freed from any cold mark.